baremetal: integrate automated (re-)provisioning logic

[pothos]

• baremetal: introduce CLC snippets for the installer
  The installer OS needs to be able to get its network configuration as
  CLC snippet if DHCP is not used.

  Add CLC snippets for the installer and migrate the matchbox-specific
  CLC template to the normal CLC template which is needed for CLC
  snippets. The request template variable is replaced by the actually
  used "mac" parameter.

• baremetal: use ignition.config.url kernel parameter
  The Ignition configuration currently gets written out only once to the
  OEM partition and any updates to it in the matchbox store are not
  propagated to the node's Ignition environment when Ignition is forced
  to run again.

  Use the Ignition kernel parameter to point the node's Ignition
  environment directly to matchbox where the userdata lies.

• baremetal: integrate automated (re-)provisioning logic
  The bare-metal platform currently lacks any understanding of whether an
  instance is actually running the configuration that lokoctl put into
  matchbox, because, when the configuration was updated, there is no
  notification to the user that PXE booting has to be done again for this
  instance.
  Also, it is not clear to the user when to boot from PXE because the PXE
  boot must happen after lokoctl populated matchbox with the (new)
  configuration but before any other steps time out.
  The goal of this patch is to bring the baremetal platform to an
  actually usable level and support automated provisioning and
  reprovisioning, in a configurable way regardless if IPMI is used or VMs
  are created. In addition, we don't want to require a complicated PXE
  boot for each configuration update because it is slow, fragile and
  needs a special DHCP infrastructure which may be lacking in a
  production environment.

  Add user-defined commands in the "pxe_commands" variable to perform
  automated PXE provisioning at the right time, i.e. initally at the
  first run or when recreating a node.
  To address the problem that PXE booting is a long and maybe even manual
  process, or even impossible at production side, we can rely on Ignition
  to simulate reprovisioning by creating the "first_boot" flag file via
  SSH and issuing a reboot, which makes Ignition fetch the configuration
  from matchbox, and if we make sure to clean the root filesystem by
  formatting it, the result is the same as if reprovisioned was done with
  a PXE boot.
  The logic is achieved by a "null resource" in Terraform that executes a
  helper script which either does a PXE boot or uses SSH to trigger a
  reprovisioning with Ignition. It also handles the case of ignoring
  userdata changes for controller nodes to prevent losing etcd state.
  Since there is no notion of a baremetal node on the Terraform level
  (reminder: all this exercise here is done because we don't have a
  Terraform provider doing this for us) a local flag file is created
  under the asset directory on the machine which runs lokoctl. If it
  exists, the node was provisioned with PXE and SSH will be used for
  reprovisioning, if it does not exist, it will be provisioned with PXE
  during inital setup and for the next reprovisioning because the user
  forced recreating the node by deleting the flag file.
  Another flag file on the node is used to check whether a node was
  successfully reprovisioned.
  When SSH is used to reprovision, the kernel parameters for GRUB are
  updated directly because they are not part of the Ignition
  configuration.
  The "copy-controller-secrets" step is run after recreating a controller
  node, again since there is no notion of a node object this is solved by
  depending on the variables itself which define the node state.
  Also add user-defined commands in the "install_pre_reboot_cmds"
  variable to run after the PXE OS installation and before booting into
  the final OS, needed to set up persistent booting from disk after the
  PXE booting was configured in "pxe_commands".
  The whole patch is used by Racker (https://github.com/kinvolk/racker),
  and can be tested either with the "bootstrap/prepare.sh" script to
  create VMs with lokoctl or by running Racker in the QEMU IPMI simulator
  environment through the "racker-sim/ipmi-env.sh" script and a Racker
  Docker image built with "installer/conf.yaml" pointing to this
  Lokomotive branch.

[ipochi]

If you can provide some more explanation that would be easy for others to review.

[ipochi]

lgtm